Diagnostic Challenges in a Young Man with a Suspected Mast Cell Disorder, Dysplastic Bone Marrow Morphology, and a ZRSR2 Mutation

<b>Background and Clinical Significance:</b> Mastocytosis and mast cell activation syndrome (MCAS) include conditions in which patients manifest signs, symptoms, and laboratory findings consistent with mast cell activation and can only be diagnosed in the presence of specific criteria. Mutations of <i>ZRSR2</i>, a gene involved in RNA splicing, are not closely associated with mast cell disorders, but rather with myelodysplastic syndromes development. <b>Case Presentation:</b> We report a case of a 37-year-old man who was referred to our institution for anaphylaxis after a bee sting and elevated serum tryptase levels (17.8 ng/mL in the first sample and 19.2 ng/mL in the second sample). Complete blood count was unremarkable. Bone marrow biopsy showed signs of dysplasia and some CD25+ mast cells. ASO-qPCR and targeted myeloid NGS analysis did not detect the <i>KIT</i> p.D816V mutation, but rather showed the presence of a pathogenetic variant of the <i>ZRSR2</i> gene (p.S447_R448del) with a variant allele frequency of 7.4%. Mastocytosis could not be diagnosed based on the established diagnostic criteria. The patient's symptoms were not recurrent and tryptase release was not event-related; therefore, a diagnosis of MCAS could not be made either. Taken together, these findings led to the diagnosis of clonal hematopoiesis of indeterminate potential (CHIP). A watch and wait strategy consisting of clinical evaluations, blood tests, and cardiovascular risk assessment was initiated. <b>Conclusions:</b> This case report highlights the importance of combining clinical and laboratory findings, hematopathology, and molecular analyses to establish the most probable diagnosis in challenging cases. It also underscores the possible relevance of identifying predisposing conditions, such as CHIP, in order to guide counseling and follow-up strategy.